1. Field of the Invention
This invention relates to a system for determining deterioration of a catalyst arranged in the exhaust system of an internal combustion engine for purifying exhaust gases emitted from the engine, and more particularly to such a system which determines deterioration of the catalyst by the use of an output from at least one oxygen concentration sensor arranged in the exhaust system.
2. Prior Art
Conventional methods for determining deterioration of catalysts for purifying exhaust gases from internal combustion engines include a method which comprises providing O.sub.2 sensors (oxygen concentration sensors) arranged upstream and downstream of a catalyst arranged in the exhaust system of an internal combustion engine, changing the air-fuel ratio of a mixture supplied to the engine, and measuring a time period elapsed from the time the air-fuel ratio is changed to the time the output from the O.sub.2 sensor arranged downstream of the catalyst changes from a leaner side to a richer side with respect to a stoichiometric air-fuel ratio or vice versa (e.g. Japanese Provisional Patent Publications (Kokai) Nos. 2-30915, 2-33408, and 2-207159), a method which is similar to the above-mentioned method but comprises measuring a first time period elapsed from the time the air-fuel ratio is changed to the time the output from the O.sub.2 sensor arranged upstream of the catalyst changes from the leaner side to the richer side or vice versa and a second time period elapsed from the air-fuel ratio is changed to the time the output from the downstream O.sub.2 sensor is changed from the leaner side to the richer side or vice versa, and measuring a time difference between the first and second time periods (e.g. Japanese Provisional Patent Publication (Kokai) No. 2-310453).
Most of these conventional or known methods include calculating the sum or average value of the time period elapsed from the time the air-fuel ratio is changed to the time the downstream O.sub.2 sensor output changes from the leaner side to the richer side with respect to the stoichiometric air-fuel ratio and the time period elapsed from the time the air-fuel ratio is changed to the time the O.sub.2 sensor output changes from the richer side to the leaner side, or the sum or average value of the time differences between the first and second time periods, and using the calculated sum or average value for determination of deterioration of the catalyst, to thereby ensure accurate determination of the catalyst deterioration.
Further proposed methods for determining deterioration of catalysts include a method which comprises comparing between an output from the upstream O.sub.2 sensor and an output from the downstream O.sub.2 sensor, such as a method of determining the ratio between the two sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 63-231252), a method of determining the response ratio between the sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 3-57862), and a method of determining the phase difference time between the sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 2-310453).
Further, a method of this kind has been proposed by U.S. Ser. No. 07/694,831 assigned to the assignee of the present application, which comprises switching an air-fuel ratio correction coefficient at a constant frequency, to determine an area difference between an output from the upstream O.sub.2 sensor and an output from the downstream sensor, and determining deterioration of the catalyst from the determined area difference (area difference method).
However, some of the above-mentioned prior art methods employ comparison between the outputs from the upstream and downstream O.sub.2 sensors. As a result, they can suffer from errors in the result of deterioration determination, which are due to variations in the characteristics of O.sub.2 sensors used and/or aging thereof. Particularly, the upstream O.sub.2 sensor, which is directly exposed to hot exhaust gases, undergoes faster deterioration than the downstream O.sub.2 sensor. The rate at which the determination proceeds is thus different between the upstream and downstream O.sub.2 sensors, which results in an error in the result of deterioration determination.
Also the area difference method proposed by the present assignee, referred to above, suffers from an error in the result of deterioration determination due to deterioration of the upstream O.sub.2 sensor. Another disadvantage of this method is that when the central value of the air-fuel ratio correction coefficient subjected to switching deviates from a value corresponding to a stoichiometric air-fuel ratio, it causes changes in the outputs from the O.sub.2 sensors, resulting in a variation in the determined area difference.
Further, all the above-mentioned prior art methods do not fully contemplate characteristics or action of the catalyst which are related to its O.sub.2 storage capacity, and therefore are unable to accurately determine deterioration of the catalyst. More specifically, in the prior art methods, measurement of the time period or the time difference elapsed or obtained at the time the downstream O.sub.2 sensor output changes from the leaner side to the richer side and measurement of the time period or the time difference elapsed or obtained at the time the downstream O.sub.2 sensor output changes from the richer side to the leaner side are made independently of each other, i.e. in a manner not related to each other. This results in inaccurate determination of deterioration of the catalyst.
Still further, the O.sub.2 storage capacity of catalysts in general varies with a change in the temperature of the catalyst, which causes a variation in the time period (CTL) elapsed before the downstream O.sub.2 sensor output changes from the richer side to the leaner side, also resulting in a low degree of accuracy of determination of the catalyst deterioration.